feat(server-mode): spend quota per user — no new session, prompts refused once used up

An admin sets a default spend quota on the Users page and overrides it per
user (Edit user → Spend quota: default / no limit / their own), in dollars
per local day or per sliding 5 hours. Spend is the users page's own
hourlyTokenRows + calculateCost over the sessions a user owns.

Over the quota, until it opens again:
- spawnRun refuses every start/resume/home prompt with EQUOTA (429);
- a minute sweep writes each pane's AGENTHUB_QUOTA_FILE (host: data dir,
  member: handoff dir via write-as) and hook-handler.js exits 2 on
  UserPromptSubmit from that file alone (Claude Code, Codex with trusted
  hooks) — fail-open, never asks the dashboard, never for Antigravity;
- a member's omp on the shared login loses its gateway key (clientsFor);
- panes no hook can stop (Antigravity, Cursor, Codex without trusted
  hooks, omp on its own login) are ended.

GET /api/me/quota feeds a "Quota N%" chip in the status bar.
New: server/lib/user-quota.js, user_quotas table, PUT /api/users/quota-default,
PATCH /api/users/:id {quota}, docs in SERVER-MODE.md § Spend quotas.

client/src/components/__tests__/StatusBar.test.tsx

@@ -129,6 +129,20 @@ vi.mock("../../lib/api", () => ({
         daily_costs: [],
       })),
     },
+    me: {
+      // Server mode: this user's own spend quota, used up.
+      quota: vi.fn().mockResolvedValue({
+        quota: {
+          limit_usd: 10,
+          period: "day",
+          spend: 12,
+          pct: 120,
+          exceeded: true,
+          resetsAt: "2026-10-29T17:00:00.000Z",
+          since: "2026-10-28T17:00:00.000Z",
+        },
+      }),
+    },
     budgets: {
       status: vi.fn().mockResolvedValue({
         budgets: [
@@ -183,6 +197,16 @@ describe("StatusBar", () => {
     expect(chip.getAttribute("title")).toBe("Monthly: $85.00 of $100.00 this period");
   });
 
+  it("shows the user's own spend quota, red once used up", async () => {
+    at("/", <StatusBar wsConnected left="0" onConnectionClick={() => {}} />);
+    const chip = await screen.findByTestId("status-bar-user-quota");
+    expect(chip.textContent).toBe("Quota 120%");
+    expect(chip.className).toContain("text-status-danger");
+    expect(chip.getAttribute("title")).toMatch(
+      /^Your spend quota is used up \(\$12\.00 of \$10\.00 today\)/
+    );
+  });
+
   it("says Disconnected when the socket is down", () => {
     at("/", <StatusBar wsConnected={false} left="0" onConnectionClick={() => {}} />);
     expect(screen.getByRole("button", { name: /Disconnected/ })).toBeInTheDocument();